Short backfire antennas (SBFAs) have seen wide use in terrestrial, maritime, and space-based applications due to their high directivity and low profile. Compared to endfire elements such as the Yagi and Helix antennas, the height of the SBFA is approximately ⅛ of Yagi and ⅕ of Helix antennas for the same directivity (e.g., about 15 dBi). One of the simplest and most widely used variations of the SBFA includes a shallow half-cylinder reflector with a 2λ diameter and a 0.25λ high rim. This SBFA is fed by a dipole placed 0.2λ above the center of the back wall of the reflector, and has a 0.4λ sub-reflector placed 0.25λ above the dipole. The polarization can be linear or circular. The measured antenna efficiency of this SBFA is approximately 83.9% (15.2 dBi). One variation of this basic configuration replaces the flat main reflector disc with conical profile, and also adds a small parasitic sub-reflector. This type of antenna has similar efficiency to the above-described SBFA with shallow half-cylinder reflector but with a wider bandwidth.
Another variation is an archery target antenna that uses an annular ring around the sub-reflector, allowing the antenna to use a much larger 5λ main reflector at the expense of approximately 46% aperture efficiency. An additional variation employs annular corrugated soft surface walls to improve the directivity over a baseline configuration with straight metal walls. However, both versions exhibited relatively low aperture efficiency.
For SBFAs, linear polarization (LP) can be generated by a linearly polarized feed such as a dipole or LP microstrip patch antenna, and circular polarization (CP) may be generated by a circularly polarized feed such as a crossed dipole fed via 90° hybrid, a CP microstrip patch antenna, or a spiral feed. The circular polarization can also be generated by a linearly polarized feed with a planar (spatial) CP polarizer in the aperture such as a meander-line polarizer.